The invention relates generally to the field of image capture, and more specifically to an image sensing device with different color types of photosites.
Electronic image sensing devices have employed a technique of sparsely sampling photographic images with an arrangement of photosites having different color responses to light. The term sparsely sampling as used herein refers to the fact that all colors are not sensed at all photosites in the image sensing device. In 1975 Bayer disclosed a sparsely sampled image sensing device in the form of a color filter array of photosites in U.S. Pat. No. 3,971,065 issued Jul. 20, 1976 to Bayer. The image sensing device described by Bayer was configured in a rectangular array of photosites with luminance sensitive photosites occurring every other photosite of every row of photosite with the pattern of luminance sensitive photosites staggered by one pixel in adjacent rows of photosite. Thus the luminance sensitive photosites formed a checkerboard pattern. The other photosites of Bayer""s image sensing device used two types of chrominance, or color, sensitive photosites with a first chrominance photosite type used in the odd rows of photosites, and a second chrominance photosite type used in the even rows of photosites.
Bayer also disclosed an alternative configuration for a sparsely sampling image sensing device in U.S. Pat. No. 3,971,065 that over time has proven to be more useful. In this alternative configuration, green sensitive photosites are used to sense the luminance signal and red and blue sensitive photosites are used for the two chrominance types of photosites as shown in FIG. 2a. The advantage of Bayer""s alternative configuration is a sensor with nearly equalized photo response between the three types of photo elements, i.e. the three types of photosites saturate at nearly the same incident light level when exposed to white light. A disadvantage of both the Bayer configurations is evident when over exposure conditions arise. Since the green sensitive photosites have a closer relationship to the image luminance signal, many real world objects have more green image content than red or blue image content. Consequently, if a given scene is overexposed, the green sensitive photosites generally saturate (reach the maximum possible signal strength) faster than the red or blue sensitive photosites. The resulting pixel values corresponding to the saturated photosites are clipped (reach a maximum pixel value) resulting in a loss of spatial detail. FIG. 2b shows the Bayer pattern with X""s at the locations of the saturated green photosites. These saturated green photosites may obscure image detail. Not only is there a loss of spatial detail but there is also a loss of green color information at these photosites.
U.S. Pat. No. 5,323,233 issued Jun. 21, 1994 to Yamagami et al. discloses an image sensing device color filter array of photosites with a different configuration as a variation on Bayer""s pattern. The Yamagami pattern of photosites is shown in FIG. 3a with the Y labeled photosites responding to luminance light, and R, G, and B labeled photosites responding to red, green, and blue light respectively. The Yamagami pattern contains 50% luminance photosites, 25% green photosites, 12xc2xd% red photosites, and 12xc2xd% blue photosites. The Yamagami pattern for overexposed conditions is shown in FIG. 3b with saturated luminance photosites indicated by X""s. The red, green, and blue photosites may still produce modulated signals. However, for overexposed conditions the Yamagami pattern does suffer a loss of spatial resolution due to the fact that 50% of the array are luminance response photosites.
There is a need therefore for an improved pattern of photosites for a image sensing device that employs a sparsely sampling technique.
The need is met according to the present invention by providing an image sensing device having an array of light-sensitive elements, including a first type of element sensitive to a spectral region corresponding to luminance; a second type of element sensitive primarily to red light; a third type of element sensitive primarily to green light; and a fourth type of element sensitive primarily to blue light, the four types of elements occurring in repeating patterns. In a preferred embodiment, the four types of elements of the image sensing device which occur in repeating patterns can be configured such that over at least a major portion of the array all four types of elements occur at every other element position along both of two orthogonal directions of the array of light-sensitive elements.
An advantage of the present invention that the image sensing device minimizes the loss of spatial detail for overexposed conditions.